U.S. patent number 9,217,861 [Application Number 14/373,071] was granted by the patent office on 2015-12-22 for micro-mirror arrays.
This patent grant is currently assigned to IMEC VZW. The grantee listed for this patent is IMEC VZW. Invention is credited to Murali Jayapala, Veronique Rochus, Xavier Rottenberg, Simone Severi, Geert Van der Plas.
United States Patent |
9,217,861 |
Jayapala , et al. |
December 22, 2015 |
Micro-mirror arrays
Abstract
Micro-mirror arrays configured for use in a variable focal
length lens are described herein. An example variable focal length
lens comprises a micro-mirror array having a plurality of
micro-mirror elements arranged in at least a first section and a
second section. Each micro-mirror element has a tilt axis and
comprises, on each of two opposing sides of the tilt axis, (i) at
least one actuation electrode, (ii) at least one measurement
electrode, and (iii) at least one stopper. Additionally, each
micro-mirror element in the first section has a first tilt angle
range, and each micro-mirror element in the second section has a
second tilt angle range, with the first tilt angle range being less
than the second tilt angle range.
Inventors: |
Jayapala; Murali (Leuven,
BE), Van der Plas; Geert (Leuven, BE),
Rochus; Veronique (Leige, BE), Rottenberg; Xavier
(Kessel-Lo, BE), Severi; Simone (Leuven,
BE) |
Applicant: |
Name |
City |
State |
Country |
Type |
IMEC VZW |
Leuven |
N/A |
BE |
|
|
Assignee: |
IMEC VZW (Leuven,
BE)
|
Family
ID: |
47722221 |
Appl.
No.: |
14/373,071 |
Filed: |
January 18, 2013 |
PCT
Filed: |
January 18, 2013 |
PCT No.: |
PCT/EP2013/050982 |
371(c)(1),(2),(4) Date: |
July 18, 2014 |
PCT
Pub. No.: |
WO2013/107890 |
PCT
Pub. Date: |
July 25, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20140368920 A1 |
Dec 18, 2014 |
|
Foreign Application Priority Data
|
|
|
|
|
Jan 20, 2012 [EP] |
|
|
12152011 |
Jan 20, 2012 [EP] |
|
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12152013 |
Jan 20, 2012 [EP] |
|
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12152015 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
27/10 (20130101); G02B 26/0833 (20130101); G02B
26/08 (20130101); H04N 13/322 (20180501); G02B
17/0896 (20130101); G02B 7/182 (20130101); H04N
13/365 (20180501); G02B 26/0825 (20130101); G02B
5/08 (20130101); G02B 26/0841 (20130101) |
Current International
Class: |
G02B
26/00 (20060101); G02B 27/10 (20060101); H04N
13/04 (20060101); G02B 17/08 (20060101); G02B
5/08 (20060101); G02B 26/08 (20060101); G02B
7/182 (20060101) |
Field of
Search: |
;359/290-295,298,237,625-627,846,850,854,855,872 ;324/679 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
International Search Report and Written Opinion, PCT International
Application No. PCT/EP2013/050982, dated Jul. 30, 2013. cited by
applicant.
|
Primary Examiner: Ben; Loha
Attorney, Agent or Firm: McDonnell Boehnen Hulbert &
Berghoff LLP
Claims
The invention claimed is:
1. A variable focal length lens comprising: a micro-mirror array
having a plurality of micro-mirror elements arranged in at least a
first section and a second section, wherein: each micro-mirror
element has a tilt axis and comprises, on each of two opposing
sides of the tilt axis, (i) at least one actuation electrode, (ii)
at least one measurement electrode, and (iii) at least one stopper;
each micro-mirror element in the first section has a first tilt
angle range; each micro-mirror element in the second section has a
second tilt angle range; and the first tilt angle range is less
than the second tilt angle range.
2. The variable focal length lens according to claim 1, wherein:
the at least one stopper of each micro-mirror element in the first
section has a first height, the at least one stopper of each
micro-mirror element in the second section has a second height, and
the first height is greater than the second height.
3. The variable focal length lens of claim 2, wherein, for each
micro-mirror element in at least one of the first section or the
second section, the tilt axis is asymmetrical.
4. The variable focal length lens of claim 3, wherein, for each
micro-mirror element in at least one of the first section or the
second section, a height of the at least one stopper on one side of
the tilt axis is greater than a height of the at least one stopper
on an opposite side of the tilt axis.
5. The variable focal length lens of claim 1, wherein, for each
micro-mirror element, the at least one actuation electrode and the
at least one measurement electrode on each of the opposing sides of
the tilt axis provide asymmetrical voltage sensitivity about the
tilt axis.
6. The variable focal length lens of claim 1, wherein: the
plurality of micro-mirror elements is arranged as a polar grid, the
first section comprises an innermost section of the polar grid, and
the second section comprises an outermost section of the polar
grid.
7. The variable focal length lens of claim 6, wherein: the polar
grid comprises a plurality of additional sections between the first
section and the second section, the tilt angle range of each
micro-mirror element in each section differs from the tilt angle
range of each micro-mirror element in each adjacent section, and
the tilt angle range progressively increases from the first section
to the second section.
Description
FIELD OF THE INVENTION
The present invention relates to improvements in or relating to
micro-mirror array devices, and is more particularly, although not
exclusively, concerned with sectioned micro-mirror arrays for use
as a variable focal length lens.
BACKGROUND TO THE INVENTION
Micro-mirror array devices are devices comprise a plurality of
microscopically small mirrors arranged in an array. Such devices
comprise micro-electromechanical systems (MEMS) devices whose
states are controlled by a voltage between electrodes located
around the array.
Micro-mirror array devices are operated to tilt along a certain
axis (or axes) in order to deflect incident light. Typically, the
tilt of the micro-mirror is controlled by the actuation of
electrodes associated with the micro-mirror, for example, by using
an applied voltage.
Characterisation of voltage against tilt angle for a given
micro-mirror device is important in evaluating its performance.
Furthermore, this relationship of voltage against tilt angle is
also important in calibrating a micro-mirror for use in a certain
application, for example, in "smart" lenses where micro-mirrors are
used with variable focal length lenses and/or zoom lenses. In
addition, obtaining or characterising the voltage-tilt angle
relationship at run-time is often desirable to support run-time
calibration.
US-A-2008/0137173 discloses a discretely controlled micro-mirror
array device including a plurality of micro-mirrors in the form a
micro-mirror array and a substrate including control circuitry.
Each micro-mirror comprises a structure having a reflective surface
with a plurality of segmented electrodes arranged on the substrate,
the segmented electrodes being arranged to be evenly distributed or
unevenly distributed with respect to their associated micro-mirror.
Each micro-mirror is capable of both rotational and translational
movement with multiple degrees of freedom. The activation of the
segmented electrodes attracts different portions of the
micro-mirror structure to provide a desired surface profile.
However, by segmenting the electrodes and therefore increasing the
number of electrodes to control each micro-mirror, more complex
electronic circuits are required to actuate and control each of the
micro-mirror elements to provide the desired surface profile.
WO-A-2009/032347 describes a micro-mirror array device comprising a
plurality of micro-mirror array elements. Electrodes associated
with the micro-mirror elements are shaped to act as stoppers to
limit the movement of the micro-mirror elements when actuated by an
applied voltage.
By using the electrodes as stoppers, charge build-up becomes a
problem during operation of the micro-mirror.
In current micro-mirror array devices, all the micro-mirror
elements in the array are typically identical, with the one
micro-mirror element being optimised and then replicated throughout
the entire array. Micro-mirror array devices designed this way have
the disadvantage that the accuracy of tilt angle required for some
implementations is quite high and a complicated and precise
manufacturing process is needed to achieve the desired high
resolution.
In addition, although each micro-mirror element is designed to be
symmetrical about its pivot point or tilt axis, in many
applications, the micro-mirror element has asymmetrical performance
about its pivot point or tilt axis. This asymmetry cannot be
adjusted when each micro-mirror element is designed to be the
same.
Moreover, in many implementations of micro-mirror arrays, the
electrodes are used for actuation and measurement, and as described
above, in some cases, as stoppers to limit the range of movement of
each micro-mirror element.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
improved micro-mirror array device configured as a variable focal
length lens which has regions of different properties.
In accordance with one aspect of the present invention, there is
provided a variable focal length lens comprising a micro-mirror
array having a plurality of micro-mirror elements arranged in at
least two sections, each section having at least one property that
is different to that of at least one other section, each
micro-mirror element having a tilt axis and at least one actuation
electrode arranged on each side of the tilt axis; characterised in
that each micro-mirror element further comprises at least one
measurement electrode arranged on each side of the tilt axis and at
least one stopper arranged on each side of the tilt axis, said at
least one different property of said one section comprising a first
tilt angle range and said different property of said at least one
other section comprising another tilt angle range.
By having more than one section in the array, each section can be
optimised for its particular function. In particular, the tilt
angle can be more accurately controlled to provide a lens of high
resolution.
The first tilt angle range may be defined by stoppers having a
first height (h.sub.1), with the second tilt angle range being
defined by stoppers having a second height (h.sub.2) where the
first height is greater than the first second height.
By having different stopper heights for different sections of the
micro-mirror array, it is possible to optimise each section without
increasing the complexity of the electronics required to tilt each
micro-mirror element within the array.
In one embodiment, each micro-mirror element within a region has
tilt asymmetry about its tilt axis provided by stoppers of
different heights on either side of the tilt axis.
Preferably, each stopper has a conductive coating to eliminate
drift due to charge build up on each micro-mirror element during
operation.
In addition, each micro-mirror element may have sensitivity
asymmetry about its tilt axis provided by the separate actuation
and measurement electrodes on each side of the tilt axis.
Sensitivity asymmetry can be implemented by having actuation
electrodes on one side of the tilt axis that are different to the
actuation electrodes on the other side of the tilt axis, for
example, by having electrodes that are of different size and/or
shape or a different number of electrodes.
In one embodiment, each micro-mirror element may further comprise
at least two measurement electrodes, one measurement electrode
being located on one side of the tilt axis and another measurement
electrode being located on the other side of the tilt axis.
By separating the actuation and measurement electrodes, each
electrode can be optimised for its particular function and there is
no need to have a single electrode which is a compromise to allow
for both actuation and measurement.
In a particular embodiment, the plurality of micro-mirror elements
is arranged as a polar grid, the polar grid being divided into at
least an inner region and an outer region, the micro-mirror
elements of the inner region having a tilt angle that is less than
the tilt angle of the micro-mirror elements in the outer region.
The polar grid may comprise a plurality of regions extending
outwards from an innermost region to an outermost region, each
region comprising a plurality of micro-mirror elements having
different tilt characteristics to micro-mirror elements in adjacent
regions, the innermost regions having the lowest tilt angles and
the outermost region having the greatest tilt angles.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, reference will
now be made, by way of example only, to the accompanying drawings
in which:--
FIG. 1 illustrates a variable focal length lens in accordance with
the present invention;
FIG. 2 is a schematic illustration of a conventional micro-mirror
device in a neutral position;
FIGS. 3 and 4 are similar to FIG. 2 but illustrate the micro-mirror
device in a first and a second tilt position respectively;
FIG. 5 illustrates a graph of capacitance against tilt angle for
the device of FIGS. 2 to 4;
FIG. 6 is a schematic illustration of one configuration of a
micro-mirror device;
FIGS. 7 and 8 are similar to FIG. 6 but illustrate the micro-mirror
device in a first and a second tilt position respectively;
FIG. 9 illustrates a graph of capacitance against tilt angle for
the device of FIGS. 6 to 8;
FIG. 10 is a schematic illustration of a micro-mirror element used
in one region of the device of FIG. 1;
FIG. 11 is similar to FIG. 10 for another region of the device of
FIG. 1;
FIG. 12 illustrates a voltage-tilt angle characteristic for the
micro-mirror elements shown in FIGS. 10 and 11;
FIG. 13 is similar to FIGS. 10 and 11 but illustrates a
micro-mirror element with different electrode heights;
FIG. 14 illustrates a voltage-tilt angle characteristic for the
micro-mirror element shown in FIG. 13;
FIG. 15 is similar to FIGS. 10 and 11 but illustrates a
micro-mirror element having a different number of electrodes on
either side of the pivot point or tilt axis;
FIG. 16 illustrates a voltage-tilt angle characteristic for the
micro-mirror element shown in FIG. 15;
FIG. 17 is similar to FIGS. 10 and 11 but illustrates a
micro-mirror element having electrodes of different shapes;
FIG. 18 illustrates a voltage-tilt angle characteristic for the
micro-mirror element shown in FIG. 17;
FIG. 19 is similar to FIGS. 10 and 11 but illustrates a
micro-mirror element having separate measurement electrodes;
FIG. 20 is similar to FIG. 19 but illustrates a micro-mirror
element having different stopper heights;
FIG. 21 illustrates a voltage-tilt angle characteristic for the
micro-mirror element shown in FIG. 20;
FIG. 22 illustrates a graph of capacitance against tilt angle
characteristic for a micro-mirror element;
FIG. 23 is similar to FIG. 23 but illustrates the effect of
decoupling the functionality of the stoppers from that of the
electrodes;
FIG. 24 is similar to FIG. 22 but illustrates the effect of stopper
position on linearity;
FIG. 25 is similar to FIG. 24 but illustrates an increase of
linearity within the effective operating area; and
FIG. 26 illustrates a graph illustrating performance asymmetry of a
symmetrically designed micro-mirror element.
DESCRIPTION OF THE INVENTION
The present invention will be described with respect to particular
embodiments and with reference to certain drawings but the
invention is not limited thereto. The drawings described are only
schematic and are non-limiting. In the drawings, the size of some
of the elements may be exaggerated and not drawn on scale for
illustrative purposes.
It will be understood that the terms "vertical" and "horizontal"
are used herein refer to particular orientations of the Figures and
these terms are not limitations to the specific embodiments
described herein.
The present invention relates to a variable focal length lens
comprising a plurality of micro-mirror elements, each micro-mirror
element being optimised for its particular performance. By
decoupling the functionality of actuation, measurement and tilt
angle, it is possible to obtain a more linear performance for each
micro-mirror element in the array forming the variable length focal
length lens.
FIG. 1 illustrates a plan view of a polar grid micro-mirror array
100 that can be configured for use as a variable focal length lens.
The micro-array 100 comprises a plurality of micro-mirror elements
arranged in eight concentric rings 110, 120, 130, 140, 150, 160,
170, 180 arranged around a central micro-mirror element 190. In
this array, each ring 110, 120, 130, 140, 150, 160, 170, 180
comprises a different number micro-mirror elements as shown and the
illustrated array 100 comprises one hundred and nine micro-mirror
elements.
However, it will be appreciated that the array may comprise any
suitable number of micro-mirror elements arranged in a regular or
irregular pattern within the array. In addition, the array is not
limited to a polar grid array. Moreover, groups of elements within
the array can operate as individual sections, the elements within
each section having substantially the same properties. The
properties of each section may be the same or different to other
sections within the array.
In accordance with the present invention, the array 100 is divided
into two sections, an outer section and an inner section as
indicated by the shading. The outer section comprises rings 110,
120, 130, 140, 150 and the inner section comprises rings 160, 170,
180 together with the central micro-mirror element 190. The maximum
tilt angle of each section is different and is controlled by
different stopper heights as described in more detail below with
reference to FIGS. 10 and 11 below.
It has been found that, for a given focal length, the tilt angle
required depends on the location of the micro-mirror element within
the micro-mirror array. In the particular embodiment described
above with reference to FIG. 1, this is a radius and the tilt
angles of micro-mirror elements at different radii is different.
Hence, the total range of tilt angles of micro-mirror elements at a
particular radius is the same and different at different radii.
It has been noted that stopper heights are important for obtaining
the desired reference tilt angles for a particular operating tilt
range. As described above, the micro-mirror elements are grouped so
that a range of radii form a section of the micro-mirror array,
that is, from the centre to a first radius, r.sub.1, for a first
section, and from the first radius, r.sub.1, to a second radius,
r.sub.2, for a second section. In this embodiment,
r.sub.1<r.sub.2 and r.sub.2 is the maximum radius of the polar
grid array 100.
A conventional micro-mirror device and its operation will be first
described with reference to FIGS. 2 to 5.
In FIG. 2, a conventional micro-mirror device 200 is shown. The
device 200 comprises a micro-mirror element 210 which is mounted at
a pivot point or tilt axis 220 about which it can tilt in
accordance with applied voltage. Two electrode elements 230, 240
are provided which are mounted on a support 250 and are equally
spaced from the pivot point or tilt axis 220. Here, the
micro-mirror element 210 is shown in a neutral or substantially
horizontal position. It will be appreciated, however, that in
certain applications, the neutral position may be at an angle to
the horizontal.
When an actuation voltage is applied to of electrode 230, as shown
in FIG. 3, the micro-mirror element 210 tilts about pivot point or
tilt axis 220 and a capacitance 260 can be measured at the other
electrode 240. This capacitance 260, when compared to the
capacitance in the neutral or previous position provides a change
in capacitance which corresponds to the tilt angle, .theta., when
measured from the horizontal as indicated by dotted line 270.
Similarly, in FIG. 4, an actuation voltage applied to the electrode
240 causes the micro-mirror element 210 to tilt about the pivot
point or tilt axis 220 and to produce a capacitance 280 that can be
measured at the electrode 230 as shown. This capacitance 280, when
compared to the capacitance in the neutral or previous position
provides a change in capacitance which corresponds to the tilt
angle, .theta.', when measured from the horizontal as indicated by
dotted line 270.
In FIGS. 3 and 4, the horizontal as indicated by dotted line 270 is
considered to be the neutral position, but it will be appreciated
that any other position can be chosen as the neutral position. In
addition, the tilt angle can be measured from a previous position
of the micro-mirror element 210 which is not horizontal.
It will be appreciated that the value of the tilt angle .theta.'
may be the same as, or different from, the value of the tilt angle
.theta.. By measuring the change in capacitance in each case, the
tilt angle of the micro-mirror element 210 due to the actuation
voltage can be determined irrespective of which of the two
electrodes 230, 240 has the actuation voltage applied to it.
In the micro-mirror device 200 illustrated in FIGS. 2 to 4, the
same electrodes 230, 240 are used for both applying the actuation
voltage and making the capacitance measurement. This means that the
two electrodes 230, 240 are effectively coupled and, in ideal
conditions, should be identical to one another.
FIG. 5 illustrates a graph of capacitance, C, against tilt angle,
.theta., for the micro-mirror device 200 shown in FIGS. 2 to 4.
Line A corresponds to the relationship between capacitance and tilt
angle for the device 200. By taking two capacitance measurements, a
change in capacitance, .DELTA.C, can be obtained and from the
position of each of the two capacitance measurements on the line A,
a change in tilt angle, .DELTA..theta., can be determined as shown.
The change in capacitance, .DELTA.C, can be determined from any two
suitable capacitance measurements which correspond to particular
tilt angles.
For a sensitive micro-mirror device, small changes in tilt angle,
.DELTA..theta., should provide large changes in capacitance,
.DELTA.C. However, the conventional micro-mirror device 200 shown
in FIGS. 2 to 4 may not be sufficiently sensitive in some
applications.
Sensitivity can be defined as a change in tilt angle for a change
voltage actuation or vice versa. For example, for a given change in
voltage applied to the an actuation electrode, if the change in
tilt angle is large then the device is considered to be sensitive.
Similarly, if the change in tilt angle is small for the same change
in voltage, then the device is considered to be less sensitive.
If the sensitivity of on one side of the conventional device is to
be reduced, due to its symmetrical arrangement with respect to the
electrodes, the sensitivity on the other side must be reduced as
well. This is due in part to a single electrode being used for both
actuation and measurement. In addition, if the sensitivity on one
side is reduced, this also has the disadvantage that the drive
electronics for one side is over-designed when compared to the
drive electronics for the other side.
It is, however, possible to decouple the two sides of the
conventional device but this requires separate actuation
mechanisms, that is, actuation circuits, which increases the
complexity of the electronics. It is therefore not possible to
alter readily the sensitivity on one side only of the micro-mirror
device without compromising on performance of the electronics.
It will be appreciated that the sensitivity of the device can be
adjusted in accordance with the particular implementation
requirements. The sensitivity can be adjusted by changing at least
one of: the size and the shape, for example, the height of the
electrodes; and the number of electrodes on each of the two sides
of the pivot point or tilt axis. By changing at least one of the
size, the shape or the number of electrodes in the micro-mirror
device, different voltage-tilt angle characteristics can be
obtained. This is described below with reference to FIGS. 13 to
18.
One way of altering sensitivity is to provide separate actuation
and measurement electrodes. By doing so, the electrodes can be
optimised for their particular operation, namely, that of being an
actuation electrode or of being a measurement electrode.
In FIG. 6, a configuration of a micro-mirror device 300 is shown in
which the separate electrodes are used for actuation and
measurement. The device 300 comprises a micro-mirror element 310
which is mounted at a pivot point or tilt axis 320 about which it
can tilt in accordance with applied voltage. Two measurement
electrodes 330, 340 are mounted on a support 350 and are equally
spaced from the pivot point or tilt axis 320. Two actuation
electrodes 360, 370 are positioned between the measurement
electrodes 330, 340 and the pivot point or tilt axis 320.
Naturally, although shown of different sizes, the measurement and
actuation electrodes may be of the same size. It is also be
appreciated that the measurement electrodes may be located nearer
to the pivot point or tilt axis 320 than the actuation
electrodes.
In the micro-mirror device 300 shown in FIG. 6, the tilt range on
each side of the pivot point or tilt axis 320 is the same, for
example, 5.degree., on each side. A voltage-tilt angle
characteristic obtained by activation of one or other of the two
actuation electrodes 360, 170 is similar to that shown in FIG. 12.
As described above, the voltage-tilt angle characteristic is
substantially linear between angles .theta..sub.s1 and
.theta..sub.s2 and passing through the origin as the maximum tilt
angles .theta..sub.s1 and .theta..sub.s2 are equal.
FIGS. 7 and 8 are similar to FIG. 6 but illustrate the application
of actuation voltages. When an actuation voltage is applied to
actuation electrode 360, as shown in FIG. 7, the micro-mirror
element 310 tilts about the pivot point or tilt axis 320 and a
capacitance 390 can be measured at the measurement electrode 340.
This capacitance 390, when compared to the capacitance in the
neutral or previous position provides a change in capacitance which
corresponds to the tilt angle, .theta..sub.1, when measured from
the horizontal as indicated by dotted line 380.
Similarly, in FIG. 8, an actuation voltage applied to actuation
electrode 370 causes the micro-mirror element 310 to tilt about the
pivot point or tilt axis 320 and to produce a capacitance 390' that
can be measured at the measurement electrode 330 as shown. This
capacitance 390', when compared to the capacitance in the neutral
or previous position provides a change in capacitance which
corresponds to the tilt angle, .theta..sub.2, when measured from
the horizontal as indicated by dotted line 380.
As described above, the horizontal, as indicated by dotted line
380, is considered to be the neutral position, but the neutral
position may be any other suitable position in accordance with the
particular implementation.
It will be appreciated that the value of the tilt angle
.theta..sub.2 may be the same as, or different from, the value of
the tilt angle .theta..sub.1 depending on whether the micro-mirror
device supports symmetrical or asymmetrical tilt angles as
described in more detail below. By measuring the change in
capacitance in each case, the tilt angle of the micro-mirror
element 310 due to an actuation voltage applied to one of the
electrodes 360, 370 can be determined by capacitance measurements
taken at the measurement electrodes 330, 340.
For actuation voltages applied to either one of the electrodes 360,
370, the corresponding capacitance measurement is determined by
either one of the measurement electrodes 330, 340. In comparison
with the conventional micro-mirror device 200 described with
reference to FIGS. 2 to 4 above, the distance between the
measurement electrode 330, 340 and the horizontal position of the
micro-mirror element 310 is reduced. Whilst this can increase the
non-linearity of the capacitance measurement, an increase in the
absolute value of the capacitance is also increased.
FIG. 9 illustrates a graph showing the relationship between
capacitance, C, and the tilt angle, .theta., for both the
micro-mirror device 200 of FIGS. 2 to 4 and the micro-mirror device
300 of FIGS. 6 to 8 where the measurement and actuation electrodes
are separate electrodes. Line A corresponds to the relationship
shown in FIG. 5 for the micro-mirror device 200 and line B
corresponds to the relationship for the micro-mirror device 300
where the measurement and actuation electrodes are separate
electrodes. Due to the decoupling of the actuation electrodes 360,
370 and the measurement electrodes 330, 340, different
relationships are obtained which provides a greater change in
capacitance, .DELTA.C.sub.2, for the change in tilt angle,
.DELTA..theta.. In this case, .DELTA.C.sub.2 is greater than
.DELTA.C.sub.1, which is the capacitance change obtained for the
conventional micro-mirror device 200.
Although FIGS. 6 to 8 illustrate a micro-mirror arrangement having
two actuation and two measurement electrodes arranged equidistant
about a single pivot point or tilt axis, it will be appreciated
that any suitable number of actuation and measurement electrodes
and associated pivot points or tilt axes can be provided. It is
essential, however, that each actuation electrode is decoupled from
each measurement electrode.
Referring now to FIG. 10, a micro-mirror element arrangement 500 is
shown that can be used for micro-mirror elements in the second or
outer section of the polar grid array 100 as described above with
reference to FIG. 1. The arrangement 500 comprises a micro-mirror
element 510 which is mounted at a pivot point or tilt axis 520
about which it can tilt in accordance with applied voltage. Two
stoppers 530, 540 are provided which are mounted on a support 550
and are equally spaced from the pivot point or tilt axis 520. Two
actuation electrodes 560, 570 are also provided on the support 550
adjacent respective ones of the stoppers 530, 540. Each stopper
530, 540 has a height h.sub.2. Here, the micro-mirror element 510
is shown in a neutral or substantially horizontal position. As
described above, other neutral positions are also possible in
accordance with a particular implementation.
FIG. 11 is similar to FIG. 10 and shows a micro-mirror element
arrangement 600 that can be used for micro-mirror elements in the
first or inner section of the polar grid array 100 as described
above with reference to FIG. 1. The arrangement 600 comprises a
micro-mirror element 610 which is mounted at a pivot point or tilt
axis 620 about which it can tilt in accordance with applied
voltage. Two stoppers 630, 640 are provided which are mounted on a
support 650 and are equally spaced from the pivot point or tilt
axis 620. Two actuation electrodes 660, 670 are also provided on
the support 650 adjacent respective ones of the stoppers 630, 640.
Each stopper 630, 640 has a height h.sub.1. Again, the micro-mirror
element 610 is shown in a neutral or substantially horizontal
position.
As above, the stopper heights h.sub.1, h.sub.2 are the same for
respective stoppers 530, 540, 630, 640, the voltage-tilt angle
characteristic for each arrangement 500, 600 is shown in FIG. 12.
In FIG. 12, .theta..sub.s1=.theta..sub.s2 where .theta..sub.s1 is
the tilt angle of stopper 530 in FIG. 10 and stopper 630 in FIG. 11
and .theta..sub.s2 is the tilt angle of stopper 540 in FIG. 10 and
stopper 640 in FIG. 11.
In FIGS. 10 and 11, a single electrode is shown on each side of the
pivot point or tilt axis, the single electrode being used for both
actuation and measurement. Naturally, such a single electrode may
be replaced with separate actuation and measurement electrodes as
will be described in more detail below with reference to FIGS. 19
and 20.
The tilt range for the first or inner section is less than the tilt
range of the second or outer section, and hence the stopper heights
in the first or inner section is greater than the stopper heights
in the second or outer section, that is, h.sub.1>h.sub.2.
The advantage of having different tilt ranges in different sections
is that more accuracy is provided for the operation of each
micro-mirror element within its tilt range. In addition, simplified
electronics can be provided, for example, micro-mirror elements in
the second or outer section can be powered using low resolution
voltage generators as the resolution of the tilt angle is lower
than the resolution of the tilt of the first or inner section.
As described above, micro-mirrors usually tilt about one or more
pivot points or tilt axes, and the mechanical properties are such
that the tilt range is symmetric along the pivot point or tilt
axis, for example, 5.degree. on either side of the pivot point or
tilt axis. However, in some implementations, the tilt range needs
to be asymmetric, for example, in a varifocal lens where different
focal length ranges are required in different areas of the lens,
for example, 5.degree. on one side of the pivot point or tilt axis
and only about 2.degree. on the other side of the pivot point or
tilt axis.
Stoppers are used to determine the tilt angle range, and, the
stopper height is inversely proportional to the tilt angle. In
addition to having different stopper heights for the micro-mirror
elements in the inner and outer sections of the polar grid array
100, it is possible for each micro-mirror element in each of the
inner and outer sections to have different stopper heights as
described below with reference to FIG. 20 below.
Where stopper heights within a single micro-mirror element are
different, the sensitivity of the micro-mirror element can be
different on one side of the pivot point or tilt axis to the
sensitivity on the other side of the pivot point or tilt axis.
In the micro-mirror device 500 shown in FIG. 10, the tilt range on
each side of the pivot point or tilt axis 520 is the same, for
example, 5.degree., on each side. A voltage-tilt angle
characteristic obtained by activation of one or other of the two
actuation electrodes 560, 570 is shown in FIG. 12. In FIG. 12, the
voltage-tilt angle characteristic is substantially linear between
angles .theta..sub.s1 and .theta..sub.s2 and passing through the
origin as the maximum tilt angles .theta..sub.s1 and .theta..sub.s2
are equal as described above. A similar voltage-tilt angle
characteristic is obtained for the micro-mirror device 600 shown in
FIG. 11 although the tilt angle is different due to the difference
between stopper height h.sub.1 of stoppers 530, 540 and stopper
height h.sub.2 of stoppers 630, 640.
However, in an asymmetric tilt situation, the tilt range on one
side of the pivot point or tilt axis is different to the tilt range
on the other side of the pivot point or tilt axis. For example, the
tilt range may be 5.degree. on one side of the pivot point or tilt
axis and, 2.degree. on the other side of the pivot point or tilt
axis. If different tilt positions are required of a particular tilt
range, for example, eight different tilt positions in the 5.degree.
range and sixteen different tilt positions in the 2.degree. range,
the sensitivity on each side of the pivot point or tilt axis needs
to be adjusted accordingly.
A micro-mirror device 700 is shown in FIG. 13 in which different
electrode heights are utilised. The device 700 comprises a
micro-mirror element 710 which is mounted at a pivot point or tilt
axis 720 about which it can tilt in accordance with applied
voltage. Two stoppers 730, 740 are provided which are mounted on a
support 750 and are equally spaced from the pivot point or tilt
axis 720. Two actuation electrodes 760, 770 are also provided on
the support 750 adjacent respective ones of the stoppers 730, 740.
Each actuation electrode 760, 770 is spaced at the same distance
from the pivot point or tilt axis 720. In this case, the stoppers
730, 740 are the same and each actuation electrode 760, 770 has the
functionality as both an actuation electrode and a measurement
electrode. In FIGS. 19 and 20 below, a micro-mirror device will be
described in which the actuation and measurement electrodes are
separate electrodes.
In FIG. 13, electrode 760 has a height e.sub.1 and electrode 770
has a height e.sub.2, where e.sub.1>e.sub.2. The different
electrode heights can be considered to correspond to different
electrode shapes.
FIG. 14 illustrates the voltage-tilt angle characteristic for the
embodiment of the micro-mirror device 700 as shown in FIG. 13. The
voltage-tilt angle characteristic has a substantially linear
profile between the origin and maximum tilt angle .theta..sub.s1
for electrode 770. Similarly, the voltage-tilt angle characteristic
has a substantially linear profile between the origin and maximum
tilt angle .theta..sub.s2 for electrode 760. However, the slope of
each part of the profile is different and therefore has different
sensitivities. For electrode 770, a small change in voltage
provides a large change in tilt angle and for electrode 760, a
small change in voltage provides a small change in tilt angle.
In FIG. 15, a micro-mirror device 800 having different
sensitivities on either side of the pivot point or tilt axis is
shown. Here, the device 800 comprises a micro-mirror element 810
which is mounted at a pivot point or tilt axis 820 about which it
can tilt in accordance with applied voltage. Two stoppers 830, 840
are provided which are mounted on a support 850 and are equally
spaced from the pivot point or tilt axis 820.
In this embodiment, two actuation electrodes 860, 870 are provided
on the support 850 between stopper 830 and the pivot point or tilt
axis 820. A single actuation electrode 880 is provided on the
support between stopper 840 and the pivot point or tilt axis 820.
The three electrodes 860, 870, 880 are of the same shape and
size.
As described above, a separate measurement electrode (not shown)
may be provided on each side of the pivot point or tilt axis
820.
A similar voltage-tilt angle characteristic to that shown in FIG.
14 is obtained for the device 800 as shown in FIG. 16. By
increasing the number of actuation electrodes on one side of the
pivot point or tilt axis, increased sensitivity can be obtained as
the same voltage is applied to each electrode, and any small
changes in voltage provide a large change in tilt angle.
FIG. 17 illustrates another micro-mirror device 900. Here, the
device 900 comprises a micro-mirror element 910 which is mounted at
a pivot point or tilt axis 920 about which it can tilt in
accordance with applied voltage. Two stoppers 930, 940 are provided
which are mounted on a support 950 and are equally spaced from the
pivot point or tilt axis 920. Two actuation electrodes 960, 970 are
also provided on the support 950 adjacent respective ones of the
stoppers 930, 940.
In this embodiment, actuation electrode 960 has a different shape
and/or size to actuation electrode 970. This difference provides
the same effect in the change of sensitivity as shown by the
voltage-tilt angle characteristic of FIG. 18. As described
previously, a separate measurement electrode (not shown) may be
provided on each side of the pivot point or tilt axis 920.
In FIG. 19, a micro-mirror device 1000 is shown for a symmetrical
tilt range. The device 1000 comprises a micro-mirror element 1010
which is mounted at a pivot point or tilt axis 1020 about which it
can tilt in accordance with applied voltage. Two stoppers 1030,
1040 are provided on a support 1050 and spaced from the pivot point
or tilt axis 1020. Between the stoppers 1030, 1040 and the pivot
point or tilt axis 1020 are located two actuation electrodes 1060,
1070 and two measurement electrodes 1080, 1090. Here, the
micro-mirror element 1010 is shown in a neutral or substantially
horizontal position.
Here, actuation electrode 1060 operates to change the tilt angle of
the micro-mirror element 1010 and measurement electrode 1090
measures the capacitance induced by the change in the tilt angle.
Similarly, when actuation electrode 1070 operates to change the
tilt angle of the micro-mirror element 1010, measurement electrode
1080 measures the capacitance induced by the change in the tilt
angle.
This means that the actuation and measurement electrodes are
decoupled and each electrode can be individually optimised for
actuation and measurement respectively. As described above, the
size, shape and number of electrodes can be modified to improve the
sensitivity.
Although the actuation electrodes are shown in FIG. 19 as being the
same, it will be appreciated they may be similar to those described
above with reference to FIGS. 13, 15 and 17 if asymmetric
sensitivity is required.
Due to the decoupling of the actuation electrodes 1060, 1070 and
the measurement electrodes 1080, 1090, different relationships are
obtained which provides a greater change in capacitance,
.DELTA.C.sub.2, for the change in tilt angle, .DELTA..theta.. In
this case, .DELTA.C.sub.2 is greater than .DELTA.C.sub.1, which is
the capacitance change obtained for the micro-mirror device 500 of
FIG. 10 (or micro-mirror device 600 of FIG. 11). This is similar to
that described above with reference to FIG. 9. As before, line A
corresponds to the relationship shown in FIG. 5 for the
micro-mirror device 500 of FIG. 10 (or micro-mirror device 600 of
FIG. 11) and line B corresponds to the relationship for the
micro-mirror device 1000.
In addition to providing a greater change in capacitance,
.DELTA.C.sub.2, decoupling the functionality of the actuation and
measurement electrodes improves the linearity of the relationship
so that relative errors are reduced.
As described above with reference to FIG. 12, a voltage-tilt angle
characteristic for a micro-mirror element arrangement is shown
where the stopper heights are the same and the tilt angle on each
side of the pivot point or tilt axis is the same.
FIG. 20 illustrates an embodiment of a micro-mirror element
arrangement 1100 which is similar to the micro-mirror element
arrangement 1000 shown in FIG. 19. The micro-mirror element
arrangement 1100 is shown in a substantially neutral position and
comprises a micro-mirror element 1110 which is mounted at a pivot
point or tilt axis 1120 about which it can tilt in accordance with
applied voltage. Two stoppers 1130, 1140 are provided on a support
1150 and spaced from the pivot point or tilt axis 1120. Between the
stoppers 1130, 1140 and the pivot point or tilt axis are located
two actuation electrodes 1160, 1170 and two measurement electrodes
1180, 1190.
Here, stopper 1130 has a height h.sub.1 and stopper 1140 has a
height h.sub.2 where h.sub.1>h.sub.2. An asymmetric voltage-tilt
angle characteristic is obtained as shown in FIG. 21. Here,
.theta..sub.s1 is less than .theta..sub.s2 due to the limitation of
tilt angle provided to the micro-mirror element 1110, for example,
.theta..sub.s1 may be 2.degree. and .theta..sub.s2 may be
5.degree..
It will be appreciated that the values of 2.degree. and 5.degree.
are given by way of example only and that other values can be
chosen in accordance with the particular implementation.
Stoppers 1130, 1140, that is, stoppers having different heights
h.sub.1, h.sub.2 can be used to provide reference tilt angles for
calibration of the micro-mirror array devices to provide
voltage-tilt angle characteristic for micro-mirror elements within
such micro-mirror array devices.
As an alternative to having stoppers of different heights, stoppers
of the same height can be used but they are spaced at different
distances from their associated pivot point or tilt axis thereby
providing a different effective height relative to the micro-mirror
element.
As described above, the relationship between capacitance and tilt
angle is linear. However, in practice, the relationship is not
linear if the stopper is not decoupled from the measurement and
actuation electrodes. In FIG. 22, a graph of capacitance against
tilt angle is shown in which the relationship for an electrode
acting as a stopper is indicated by curve 400. For comparison, a
linear relationship extending between the lower point 410 and the
upper point 420 on the curve 400 is indicated by line 430. In an
effective operating area 440, it can be seen that the portion of
the curve 400 in that area cannot be considered to be linear when
compared to line 430.
When the stoppers are decoupled from the actuation and measurement
electrodes, the situation is improved as shown in FIG. 23. In FIG.
23, the curve 400 is shown but in this case the decoupling of the
stoppers enables a substantially linear relationship within the
effective operating area 440 as shown by line 450.
In addition, the position of the stopper with respect to the tilt
axis can be used to provide different ranges of sensitivity within
the effective operating area 440. As shown in FIG. 24, the
relationship between capacitance and tilt angle is linear over the
range between approximately 0.degree. and 2.degree. for a downward
tilt angle as indicated by line 460. By moving the stopper closer
to the tilt axis, as shown in FIG. 25, the range of the linear
relationship is extended to approximately 0.degree. and 4.degree.
as indicated by line 470 within the effective operating area
440.
It has been found that charge builds up in micro-mirrors during
operation which produces voltage drifts. By providing a conductive
coating on the stoppers, a path is provided for the built up charge
to discharge. This requires that the stoppers are decoupled from
the actuation and measurement electrodes as described above.
It has been found that, by designing a micro-mirror element to be
symmetric about its pivot point or tilt axis, there is still
asymmetry in performance as shown in FIG. 26. In FIG. 26, a graph
of illustrating maximum and minimum measurements corresponding to
input voltage, curve 480 corresponds to maximum measurements and
curve 485 corresponds to minimum measurements. Neither the curve
480, 485 is symmetric around the vertical axis which corresponds to
an input voltage of V.sub.in=0. In addition on the left of the
graph, for negative input voltage, there is a maximum difference of
0.29 mrad between the two curves as indicated at 490. Similarly, on
the right of the graph, for positive input voltage, there is a
maximum difference of 0.37 mrad as indicated at 495. By
characterising this information and by predicting this expected
process variation, each micro-mirror element can be designed with
asymmetric actuation electrodes so that the performance of the
micro-mirror element can be made to be symmetric. Similarly,
stoppers and/or measurement electrodes can also be optimised,
either individually or in conjunction with the actuation
electrodes, for symmetric performance.
As a result of being able to optimise each individual element
associated with a micro-mirror element to provide an asymmetric
profile, flexibility of design is provided which can compensate for
process variations.
The present invention has been described above with reference to
tilting about a single pivot point or tilt axis. However, it will
be appreciated that each micro-mirror element may tilt about more
than one pivot point or tilt axis. In this case, two actuation
electrodes, two measurement electrodes and two stoppers will be
provided for each pivot point or tilting axis.
Although the present invention has been described with reference to
two actuation and two measurement electrodes, it will be appreciate
that any suitable number of actuation and measurement electrodes
can be provided. It is essential, however, that the actuation and
measurement electrodes are decoupled from one another.
In addition, the present invention is not limited to use with a
polar grid array and can be used with any micro-mirror array where
different properties are to be provided by different sections of
the array.
Whilst the present invention has been described in relation to one
specific embodiment, it will be appreciated that modifications can
be made that fall within the scope of the present invention.
* * * * *